Stoichiometry (from the Greek stoicheion, meaning "element," and metron, meaning "measure") is the branch of chemistry dealing with the quantitative relationships between the reactants and products in a chemical reaction.

It rests on two pillars:

• The law of conservation of mass, which requires that a chemical equation be properly balanced — the number of atoms of each element must be the same on both the reactant and product sides.
• The mole concept, which allows chemists to convert between the mass of a substance and the number of particles (atoms, molecules, or ions) it contains, using molar mass as the conversion factor.

Using a balanced equation together with mole ratios, stoichiometry allows a chemist to calculate:

• How much of each reactant is required to completely react with a given amount of another reactant.
• How much of each product will be formed from a given amount of reactant(s).
• Which reactant is the limiting reagent — the one that runs out first and therefore determines the maximum amount of product that can form.
• The theoretical yield of a reaction, and by comparison with the actual yield obtained in the laboratory, the percentage yield.

This makes the correct description of stoichiometry the calculation of reactant and product amounts using balanced chemical equations.

The remaining options are each incorrect for a specific reason: stoichiometric calculations cannot ignore the mole concept, since converting masses to particle counts (and vice versa) via moles is the very mechanism by which stoichiometric calculations are carried out; stoichiometry is not concerned with physical properties (such as melting point, density, or color) of substances, since those are unrelated to reaction quantities; and stoichiometric calculations cannot be performed without a balanced equation, because an unbalanced equation would give incorrect mole ratios between reactants and products and would violate the conservation of mass on which all such calculations depend.